Conventionally, there have been used optical nonlinear evaluation devices (e.g., see Non-patent Document 1 and Patent Document 1) each of which uses a polarization Sagnac interferometer to detect, as polarization information, a phase shift caused by cross-phase modulation of a signal beam by irradiating, with a control beam, a Kerr medium that causes an optical nonlinear effect of causing a change in refractive index in accordance with light intensity.
According to the optical nonlinear evaluation device described in Non-patent Document 1, a waveguide Kerr medium such as an optical fiber is disposed on a polarization Sagnac interference path of a polarization Sagnac interferometer, and a signal beam supplied from a laser light source is split by a polarization beam splitter into a clockwise signal beam and a counterclockwise signal beam. The clockwise and counterclockwise signal beams split from each other travel along the polarization Sagnac interference path in directions opposite to each other, travel through the Kerr medium in directions opposite to each other, and then return to the polarization beam splitter. The light intensities of the signal beams traveling through the Kerr medium are differentiated from each other by an attenuator provided in the polarization Sagnac interferometer, and the phase difference thus caused between the signal beams is detected by a Babinet-Soleil compensator provided inside the polarization Sagnac interferometer and a polarizer provided outside the polarization Sagnac interferometer.
The Babinet-Soleil compensator is constituted by two wedge-shaped wave plates layered on top of each other. The phase of light traveling through the Babinet-Soleil compensator is changed by changing the way the two wedge-shaped wave plates are combined with each other. The Babinet-Soleil compensator can operate as a half-wave plate or a quarter-wave plate in accordance with the position in which the two wedge-shaped wave plates are combined with each other.
The optical nonlinear evaluation device described in Non-patent Document 1 yields an interference waveform of the signal beams by detecting the light intensities with a detector while changing the way the two wedge-shaped wave plates are combined with each other in the Babinet-Soleil compensator and then by sweeping the phases.
Further, Non-patent Document 1 proposes a measurement system obtained by replacing the polarization Sagnac interferometer with a normal Sagnac interferometer. In this measurement system, a beam splitter (BS) that splits light with a ratio of 1:1 independently of polarization is provided at an entrance of the normal Sagnac interferometer instead of the polarization beam splitter. Therefore, both the clockwise and counterclockwise signal beams in the Sagnac interferometer have the same polarization, and by causing a control beam whose polarization is orthogonal to the polarization of the signal beams to be incident, the control beam can be appropriately superimposed solely onto the clockwise signal beam.
When the control beam is not superimposed onto the clockwise signal beam, the effect of interference of light causes the signal beam incident on the normal Sagnac interferometer to be emitted to the same entrance port as it entered, and the signal beam is not emitted to the exit port. When the control beam is superimposed onto the signal beam by causing the control beam to be incident, the clockwise signal beam is subjected to cross-phase modulation. This causes a change in relative phase between the clockwise and counterclockwise signal beams, thereby causing a change in the amount of light that is to be emitted to the exit port. The phase shift can be measured by reading this change.
The optical nonlinear evaluation device described in Patent Document 1 measures the nonlinear characteristics of a Kerr medium by using signal beams. The nonlinear characteristics are caused by irradiating the Kerr medium with a control beam generated in a femtosecond region. The signal beams are generated by splitting a single optical pulse in a polarization Sagnac interferometer, and are polarized in directions orthogonal to each other. The polarization directions on a surface of the Kerr medium are aligned by a polarization direction converting mechanism including a half-wave plate that rotates the polarization directions of the signal beams 90 degrees in the polarization Sagnac interferometer. The phase difference between the signal beams outputted from the polarization Sagnac interferometer is swept by a phase difference sweeping mechanism including a quarter-wave plate and a polarizer. The nonlinear characteristics are found by measuring the intensity of light of interference between the signal beams for each phase difference that is to be swept.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 163384/2004 (Tokukai 2004-163384; published on Jun. 10, 2004)
[Non-patent Document 1]
M. C. Gabriel, et al. “Measurement of ultrafast optical nonlinearities using a modified Sagnac interferometer”, Optics Letters, Vol. 16, No. 17 (1991)